Aluminium alloy powder blends and sintered aluminium alloys

ABSTRACT

The invention relates to an aluminum powder blend and sintered components produced from the aluminum powder blend. The powder is based on the precipitation hardenable 7000 series Al-Zn-Mg-Cu alloys with trace addition of lead or tin. The powder blend comprises 2-12 wt. % zinc, 1-5 wt. % magnesium, 0.1-5.6 wt. % copper, 0.01-0.3 wt. % lead or tin, and the balance aluminum. The invention also provides a composite powder comprising the foregoing powder blend and a reinforcing element or compound.

TECHNICAL FIELD

This invention relates to an aluminium alloy powder blend for theproduction of a sintered aluminium alloy. The invention also relates tosintered aluminium alloys formed from the starting powder and articlesprepared from the sintered aluminium alloys.

BACKGROUND ART

Powder Metallurgy (P/M) is the technology of transforming metal powdersinto semi-finished or finished products by mechanical and thermaloperations. Advantages of using P/M techniques include the ability tofabricate specialty alloys with unique compositions, microstructures andproperties; to make parts of complex shape to close tolerances withoutsecondary processing; and to produce alloys, such as the refractory andreactive metals, which can only be fabricated in the solid state aspowders. Standard P/M techniques involve the pressing of metal powdersin a die, the removal of the green part from the die, and the sinteringof the part in a furnace under a controlled atmosphere. The startingpowder may be a blend of pure elemental powders, a blend of master alloypowders, fully alloyed powders or any combination thereof. Non-metallicparticulate materials may be added to make composites. The sinteringprocess causes metallic bonds to form between the powder particles. Thisprovides most of the strength. Bonding and/or densification may be aidedby the development of liquid phases during sintering. These may or maynot persist to the completion of sintering. These liquid phases may formby melting of elements or compounds, by the incipient melting ofpre-existing eutectic compounds, or by the melting of eutectics whichform by diffusional processes during sintering. The alloy may be used inthe as sintered state or may be further processed. Secondary processesinclude coining, sizing, re-pressing, machining, extrusion and forging.They may also be surface treated and/or impregnated with lubricatingliquids. Many metals are fabricated this way, including iron and steel,copper and its alloys, nickel, tungsten, titanium and aluminium.

The difficulty in sintering metal powders is a consequence of thesurface oxide film which is present on all metals. This oxide film is abarrier to sintering because it inhibits inter particle welding and theformation of effective inter particle bonds. The problem is particularlysevere in aluminium because of the inherent thermodynamic stability ofthe oxide (Al₂ O₃). Current P/M processed aluminium alloys are usedprincipally in business machines where high mechanical strength is notrequired but where low inertia and corrosion resistance are importantproperties. There is, however, a demand for high strength, pressed andsintered aluminium alloys.

A general maxim in materials engineering is that alloys are tailored tothe manufacturing process as much as to the application becausedifferent processes require different properties. Thus cast steels aredifferent to both rolled steels and P/M steels; directionally solidifiedsingle crystal nickel superalloy turbine blades have a differentcomposition to conventionally cast blades and aluminium extrusion alloysare different to forging alloys which in turn are different to castingalloys and rapidly solidified alloys. However, this principle has notyet been applied to pressed and sintered aluminium alloys. Currentcommercial alloys are predominantly based on the wrought alloys 6061 and2014, which are Al-Mg-Si and Al-Cu-Si-Mg alloys, respectively. They havenot been optimised for the P/M process.

U.S. Pat. No. 5,304,343 describes a method of producing a sinteredaluminium alloy having improved mechanical properties. However, thealloy according to this patent is made using an expensive master alloyroute and is based on 2,000 and 6,000 series alloys.

There is thus a need for an aluminium alloy powder blend, and sinteredaluminium alloys produced therefrom, which provide higher tensilestrength alloys for use in a broader range of applications than hashitherto been possible.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an aluminium alloystarting powder for manufacturing a sintered aluminium alloy havingimproved mechanical properties over previously known sintered-aluminiumalloys.

According to a first embodiment of the invention, there is provided analuminium alloy starting powder blended from pure elemental powders fora sintered aluminium alloy, said powder blend consisting essentially of2-12 wt % zinc, 1-5 wt % magnesium, 0.1-5.6% copper, 0.01-0.3 wt % leador tin, and the balance aluminium.

Preferred concentrations for the components of the powder are: zinc, 4-8wt %; magnesium, 1.5-3.5 wt %; copper 1-4 wt %; and, lead or tin,0.03-0.15 wt %.

Of the trace elements lead or tin, lead is preferred.

Typically, starting powder according to the first embodiment includes asolid lubricant such as stearic acid or waxes based on stearic acid, orother organic lubricant. A preferred solid lubricant is stearic acid inan amount between 0.1 and 2 wt %. Preferably, the stearic acid is in anamount of 0.5-1 wt %.

The size of zinc particles in the powder are advantageously of largersize than is conventionally used. Zinc particles of 60 mesh to dust inconjunction with aluminium particles of 50 mesh to dust are preferred(particle sizes by screening--ASTM E-11 mesh numbers). Other parameterssuch as heating rate and compaction pressure can be varied to enhancethe zinc size effect as will be discussed below. This aspect of theinvention is applicable to any zinc-containing aluminium alloy powderblend.

According to a second embodiment of the invention, there is provided acomposite starting powder for a sintered aluminium alloy, said powderconsisting essentially of a powder according to the first embodimenttogether with at least one reinforcing element or compound.

The reinforcing element or compound of the second embodiment istypically, but is not limited to, carbon, carborundum, corundum,titanium diboride, fly ash, cermets, silicon carbide or other oxides,carbides, nitrides and borides. In the composite powder of the secondembodiment, the reinforcement typically comprises 2 vol % to 50 vol % ofthe composite with the balance being the alloy powder of the firstembodiment. A preferred proportion of the reinforcement is 5 vol % to 30vol %.

In a third embodiment of the invention, there is provided a sinteredaluminium alloy, which alloy is produced by the steps of:

(i) compacting a powder according to the first embodiment or a compositeaccording to the second embodiment at a pressure of up to 600 MPa; and

(ii) sintering said compacted material from step (i) at a temperature of550° C. to 640° C.

In producing the sintered aluminium alloy of the third embodiment, acompaction pressure of 200 MPa to 500 MPa is preferred. Heating of thecompacted material to the sintering temperature is typically at a rategreater than 5° C./min and is preferably at a rate of between 10° C./minand 40° C./min.

The compacted material is typically held at the sintering temperaturefor not more than 2 h. Preferred sintering times and temperatures are10-30 min and 600-630° C.

The invention includes within its scope articles manufactured from thesintered aluminium alloy of the third embodiment. Articles can also bemanufactured from the sintered alloy by processes such as, but notrestricted to, powder forging or extrusion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph depicting the effect of trace additions of lead andtin on the densification of an Al-8Zn-2.5Mg-1Cu-0.07X alloy where X isthe lead or tin. Negative numbers indicate expansion; positive numbersindicate shrinkage.

FIG. 2 presents reflected light micrographs of polished sections ofsintered material showing the effect of trace additions of lead on theporosity of an Al-8Zn-2.5Mg-1Cu alloy. The material the subject of panel(a) had no trace addition while the material the subject of panel (b)contained 0.07 wt % lead. Magnification: 46X.

FIG. 3 is a graph showing the effect of trace lead addition on thetensile strength of an Al-8Zn-2.5Mg-1Cu alloy (T6 condition).

FIG. 4 is a graph of the effect of zinc particle size on the quantity ofliquid phase formed during sintering of a binary Al-10Zn alloy. Smallparticles were -325 mesh; large particles were -100 +120 mesh. Sinteringwas at 620° C. for 10 minutes. The heating rate was 10° C./min. The timeis that for which the sample was above the melting point of zinc.

BEST MODE AND OTHER MODES FOR PERFORMING THE INVENTION

As indicated above, this invention relates to the development of analuminium alloy powder blend which can be used for the manufacture ofsintered components. The sintered component can be subjected tosecondary processing operations. Specifically, this invention isconcerned with the composition of the alloy and the powder sizedistribution, particularly that of the alloying additions, whichoptimises the sintering process.

The material is based on the precipitation hardenable 7000 seriesAl-Zn-Mg-Cu alloys with trace additions of lead or tin. Lead ispreferred for the attainment of high sintered densities and henceimproved mechanical properties. Tin shows a similar but reduced effect.The addition of 100 ppm lead to an Al-8Zn-2.5Mg-1Cu alloy increases thesintered density so that the compact shrinks rather than expands duringsintering. This is illustrated in FIG. 1, while the effect on themicrostructure is shown in FIG. 2. The influence of lead on the tensilestrength is apparent from the data of FIG. 3; here the addition of 0.12wt % Pb increases the tensile strength of the Al-8Zn-2.5Mg-1Cu alloy bymore than 30%. The lead may be added as an elemental addition or it maybe pre-alloyed with the zinc.

Zinc is the principle alloying addition. Its melting point is below thesintering temperature and it forms a number of binary and ternaryeutectic phases. This should enhance sintering. However, zinc is highlysoluble in aluminium and this is an impediment to its use as a sinteringagent. When small zinc particles are used, the entire zinc addition isquickly absorbed by the aluminium and little or no liquid phases form,which hinders sintering. This has limited its previous application. Incontrast, when large zinc particles are used, the aluminium adjacent toa zinc particle becomes locally saturated and elemental zinc persistslong enough for enhanced liquid phase sintering to occur. The amount ofliquid phase formed is therefore a function of the zinc particle size.This is illustrated in FIG. 4. Because the thermodynamic driving forceis inversely proportional to the particle size and because the smallerparticle sizes aid particle packing, the zinc size needs to beoptimised. The zinc size effect is also dependent on other processvariables such as heating rate and compaction pressure. These also needto be optimised. A similar particle size effect occurs in other systemswhere there is some solid solubility of the additive in the base elementand where there is a diffusive flow from the additive to the base.Examples include copper in aluminium and copper in iron.

Magnesium is thought to disrupt the oxide film and also contributes toprecipitation hardening. Copper improves the wetting of the aluminium bythe sintering liquid, aids hardening and also improves the corrosionproperties. Both are added as pure elements. A solid lubricant, such astearic acid or waxes based on stearic acid, can be added to the powderblend to assist the compaction process. This can be removed prior tosintering by some thermal treatment or it can be removed during heatingto the sintering temperature. The alloy is sintered in a high puritynitrogen atmosphere. It can then be heat treated in the conventionalmanner for aluminium alloys.

The following table, Table I, lists typical and preferred values for thealuminium alloy powder components and values for process steps inproducing sintered alloy according to the invention. All compositionsare in weight precent and particle sizes by screening (ASTM E-11 meshnumbers).

                  TABLE I    ______________________________________                  TYPICAL       PREFERRED    PARAMETER     VALUE         VALUE    ______________________________________    Zinc concentration                  2-12%         4-8%    Magnesium concentration                  1-5%          1.5-3.5%    Copper concentration                  0.1-5.6%      1-4%    Lead or tin concentration                  0.01-0.3%     0.03-0.15%    Aluminium powder size                  -50 mesh      -100 mesh + 325                                mesh    Zinc powder size                  -60 mesh      -100 mesh    Magnesium powder size                  -100 mesh     -200 mesh    Copper powder size                  -60 mesh      -100 mesh    Compaction pressure                  50 MPa to 600 200 MPa and 500                  MPa           MPa    Heating rate  >5° C./min                                10° C./min to                                40° C./min    Sintering temperature                  550° C. to 640° C.                                600° C. to 630° C.    Sintering time                  <2 hours      10 min to 30 min    ______________________________________

The invention is further described in and illustrated by the followingexamples. These examples should not be construed as limiting theinvention is any way.

EXAMPLE 1

An alloy of 10Zn-2.5Mg-1Cu-0.09Pb-balance Al (wt %) was made by blendingelemental powders with 1 wt % stearic acid as a solid lubricant in atumbler mixer for 30 minutes. The aluminium powder was air atomised andpassed through a 60 mesh screen. A rectangular bar was made by pressingthis powder in a metal die at a pressure of 210 MPa. The zinc passedthrough a 100 mesh screen. The magnesium and the copper powder were both-325 mesh. The zinc was pre-alloyed with 0.9 wt % Pb. The green compactwas then sintered under a nitrogen atmosphere at a temperature of 600°C. for 30 minutes. It was heated to the sintering temperature at a rateof 20° C. per minute. The sample was air cooled and subsequentlysolution treated in air at 490° C. for 1 hour. A tensile specimen wasmachined from the bar. It had a tensile strength (T4 condition) of 332MPa and an elongation to failure of 1%.

EXAMPLE 2

An alloy was made as per Example 1 but with a composition of6Zn-2.5Mg-3Cu-0.05Pb-balance Al (wt %) and was sintered at 610° C. Ithad a tensile strength in the T4 condition of 312 MPa and an elongationto failure of 1.17%.

EXAMPLE 3

An alloy was made as per Example 1 but with a composition of8Zn-2.5Mg-1Cu-0.07Pb-balance Al (wt %) and a zinc particle size of -200mesh. It was heated to the sintering temperature at a rate of 5° C. perminute and sintered for 2 hours. The tensile strength in the T4condition was 328 MPa with an elongation to failure of 5.13%.

EXAMPLE 4

An alloy was made as per Example 3 but was artificially aged at 130° C.for 15 hours after solution treatment (T6 condition). The tensilestrength was 444 MPa and the elongation to failure was 1.1%.

EXAMPLE 5

An alloy was made as per Example 1 but with a composition of8Zn-2.5Mg-1Cu-0.12Pb-balance Al (wt %). Pure, un-alloyed zinc ofparticle size -325 mesh was used. Pure elemental lead (particle size-325 mesh) was added separately to the zinc. The sample was pressed at410 MPa, heated at 10° C. per minute to the sintering temperature andsintered at 600° C. for 2 hours. It was tested in the T6 condition. Thetensile strength was 424 MPa and the elongation to failure was 0.65%.

EXAMPLE 6

An alloy was made as per Example 5 but with 0.09 wt % tin replacing the0.12 wt % lead addition. The tensile strength in the T6 condition was365 MPa.

EXAMPLE 7

An alloy was made as per Example 1 but with a composition of6Zn-2.5Mg-1Cu-0.05Pb-balance Al (wt %). The aluminium had the -325 meshpowder size removed. Zinc of particle size -100 mesh and copper ofparticle size 200 mesh was used. The alloy was heated at 40° C. perminute to the sintering temperature and sintered at 620° C. for 20minutes. It had a tensile strength in the T4 condition of 304 MPa and anelongation to failure of 5.57%.

INDUSTRIAL APPLICABILITY

Alloy produced from starting powder or composite according to theinvention is suitable for manufacturing articles for use in thetechnology fields listed hereafter. The list should in no way beconsidered exhaustive and is merely provided for furtherexemplification.

1. Sintered and heat treated automotive components such as cam shaftpulleys, cam shaft and crank shaft gears, cam shaft lobes, oil pumpgears, transmission components including synchronising rings, water pumpimpellers, bearing caps and battery terminal clamps.

2. Sintered and heat treatment components for business machines andcomputer equipment such as pulleys and gears.

3. Powder forged components for high cyclic stress environments such asconnecting rods in internal combustion engines, automotive suspensionand brake components, recording heads in video and audio tape recordersand disk drive components in computers and related equipment.

It will be appreciated that many changes can be made to the alloys asexemplified above without departing from the broad ambit of theinvention, which ambit is to be limited only by the appended claims.

We claim:
 1. A starting powder blend for producing a sintered aluminumalloy, said starting powder blend consisting essentially of 2-12 wt %zinc, 1-5 wt % magnesium, 0.1-5.6% copper, 0.01-0.3 wt % lead or tin,and the balance aluminium.
 2. The starting powder according to claim 1,wherein the concentrations of said components are: zinc, 4-8 wt %;magnesium, 1.5-3.5 wt %; copper 1-4 wt %; and lead, 0.03-0.15 wt %. 3.The starting powder according to claim 1, wherein the concentrations ofsaid components are: zinc, 4-8 wt %; magnesium, 1.5-3.5 wt %; copper 1-4wt %; and tin, 0.03-0.15 wt %.
 4. The starting powder according to claim1, which further includes a solid lubricant.
 5. The starting powderaccording to claim 4, wherein said solid lubricant is stearic acid orwaxes based on stearic acid, or other organic lubricant.
 6. The startingpowder according to claim 5, wherein said solid lubricant is stearicacid at a concentration of 0.1-2 wt %.
 7. The starting powder accordingto claim 6, wherein said stearic acid concentration is 0.5-1 wt %. 8.The starting powder according to claim 1, wherein said zinc has aparticle size of 60 mesh to dust and said aluminium has a particle sizeof 50 mesh to dust.
 9. A composite starting powder for a sinteredaluminium alloy, said powder consisting essentially of a starting powderaccording to claim 1 together with at least one reinforcing element orcompound.
 10. The composite powder according to claim 9, wherein saidreinforcing element or compound is selected from carbon, carborundum,corundum, titanium diboride, fly ash, cermets, silicon carbide or otheroxides, carbides, nitrides and borides effective to providereinforcement.
 11. The composite powder according to claim 9, whereinsaid reinforcing element or compound comprises 2 vol % to 50 vol % ofthe composite with the balance said starting powder.
 12. The compositepowder according to claim 11, wherein said reinforcing element orcompound comprises 5 vol % to 30 vol % of the composite.
 13. A sinteredaluminium alloy, which alloy is produced by the steps of:(i) compactinga powder according to claim 1 or a composite according to claim 9 at apressure of up to 600 MPa; and (ii) sintering said compacted materialfrom step (i) at a temperature of 550° C. to 640° C.
 14. The aluminiumalloy according to claim 13, wherein said compaction pressure is greaterthan 50 MPa.
 15. The aluminium alloy according to claim 13, wherein saidcompaction pressure is 200 MPa to 500 MPa.
 16. The aluminium alloyaccording to claim 13, wherein said compacted material is heated to thesintering temperature at a rate greater than 5° C./min.
 17. Thealuminium alloy according to claim 16, wherein said rate is between 10°C./min and 40° C./min.
 18. The aluminium alloy according to claim 13,wherein said compacted material is held at the sintering temperature fornot more than 2 hours.
 19. The aluminium alloy according to claim 13,wherein said compacted material is held at the sintering temperature for10-30 minutes.
 20. The aluminium alloy according to claim 13, whereinthe sintering temperature is 600-630° C.
 21. An article manufacturedfrom the sintered aluminium alloy of claim 13.